A circulating water system of central air conditioning generally comprises a cooling water system, a chilled water system and a heating water system, wherein the circulating cooling water system is generally open, and the chilled water and the heating water systems are mostly enclosed. In the open circulating cooling water system, as water contacts with the atmosphere constantly, the content of dissolved oxygen increases, which is likely to cause the electrochemical corrosion; CaCO3 in water is more likely to scale and separate out on the conduction surface with the gradual increase of the salt content of the circulating water in the process of the continuous evaporation of cooling water, and with the degassing of CO2. The cooling water absorbs such a great deal of dust, sand and the like in the air while contacting with the air that the sludge in the system accumulate. In addition, lighting, right temperature and sufficient oxygen and nutrient in the cooling tower are all beneficial to the growth of bacteria and algae, and the concentration of microorganisms and their nutrient sources increase with the circulating concentration, so that slime in the system increase.
The Corrosion, scale and slime mentioned above are not isolated but interrelated with one another. For example, salt deposits and dirt are generally combined together, and corrosion can be caused or aggravated by scale and slime. Therefore, the main task of circulating cooling water treatment is to eliminate or reduce damage caused by scale, corrosion and biological slime.
Generally speaking, the water used in a central air conditioning water system is classified into three types, namely, untreated tap water, softened water and deionized water. The water quality of tap water varies from region to region. In coastal regions of South China, the water qualities of tap water are of ultra-low hardness and high corrosiveness, with water hardness less than 20 mg/L (in CaCO3, the same below), the total alkalinity less than 50 mg/L, and the pH value about 6. In softened water, however, scaling ions such as Ca2+ and Mg2+ are removed, and corrosive ions such as Na+ and Cl− are added, which will not generate the scale, but aggravate the corrosion. Although deionized water has no scaling irons or corrosive ions, it has dissolved oxygen, so the corrosion process can be slow at first and accelerates gradually.
The patent CN103420495A discloses a preparation method and application of a central air-conditioning circulating water scale inhibitor, which is a copolymer prepared by the polymerization of certain amounts of alkyl glycol ether, maleic anhydride and other polymerizable monomers. The patent CN1754844A discloses an acid antiscaling agent used in circulating water of central air-conditioning. The acid antiscaling agent comprises ethylene diamine tetraacetic acid, sodium humate, polyphosphates and water. The two inhibitors above focus on scale inhibition, and pay little attention to corrosion inhibition. The patent CN102718329A discloses a composite corrosion and scale inhibitor of ultra-low hardness circulating cooling water, and a preparation method of the composite corrosion and scale inhibitor, which consists of amino tri-methylene phosphonic acid, etidronic acid, diethylenetriaminepenta acid and the like. The patent CN103332797A discloses a phosphorus-free corrosion and scale inhibitor used in circulating cooling water and the application thereof. The phosphorus-free corrosion and scale inhibitor consists of a non-enzymatic browning reaction product, a phosphorus-free scale inhibitor, a sulfonyl copolymer, zinc salt and water. Although the above two inhibitors adopt low phosphonic or phosphonic-free treatments, their complex formulae and the operation methods fail to meet the comprehensive requirements of the circulating cooling water system on corrosion inhibition, scale inhibition, sterilization and bacteriostasis.